Frequency selection

Introduction

This chapter explains the frequency selection done on Sandy Bridge and Ivy Bridge memory initialization.

Definitions

Symbol	Description	Units	Valid region
SCK	DRAM system clock cycle time	s
tCK	DRAM system clock cycle time	1/256th ns
DCK	Data clock cycle time: The time between two SCK clock edges	s
SPD	Manufacturer set memory timings located on an EEPROM on every DIMM	bytes
REFCK	Reference clock, either 100 or 133	MHz	100, 133
MULT	DRAM PLL multiplier		[3-12]
XMP	Extreme Memory Profiles

SPD

The SPD located on every DIMM is factory program with various timings. One of them specifies the maximum clock frequency the DIMM should be used with. The operating frequency is stores as fixed point value (tCK), rounded to the next smallest supported operating frequency. Some SPD contains additional and optional XMP data, that stores so called “performance” modes, that advertises higher clock frequencies.

XMP profiles

At time of writing coreboot’s raminit is able to parse XMP profile 1 and 2. Only XMP profile 1 is being used in case it advertises:

1.5V operating voltage
The channel’s installed DIMM count doesn’t exceed the XMP coded limit

In case the XMP profile doesn’t fulfill those limits, the regular SPD will be used.

Note: XMP Profiles are supported since coreboot 4.4.

It is possible to ignore the max DIMM count limit set by XMP profiles. By activating Kconfig option NATIVE_RAMINIT_IGNORE_XMP_MAX_DIMMS it is possible to install two DIMMs per channel, even if XMP tells you not to do.

Note: Ignoring XMP Profiles limit is supported since coreboot 4.7.

Soft fuses

Every board manufacturer does program “soft” fuses to indicate the maximum DRAM frequency supported. However, those fuses don’t set a limit in hardware and thus are called “soft” fuses, as it is possible to ignore them.

Note: Ignoring the fuses might cause system instability !

On Sandy Bridge CAPID0_A is being read, and on Ivy Bridge CAPID0_B is being read. coreboot reads those registers and honors the limit in case the Kconfig option CONFIG_NATIVE_RAMINIT_IGNORE_MAX_MEM_FUSES wasn’t set. Power users that want to let their RAM run at DRAM’s “stock” frequency need to enable the Kconfig symbol.

It is possible to override the soft fuses limit by using a board-specific devicetree setting.

Note: Ignoring max mem freq. fuses is supported since coreboot 4.7.

Hard fuses

“Hard” fuses are programmed by Intel and limit the maximum frequency that can be used on a given CPU/board/chipset. At time of writing there’s no register to read this limit, before trying to set a given DRAM frequency. The memory PLL won’t lock, indicating that the chosen memory multiplier isn’t available. In this case coreboot tries the next smaller memory multiplier until the PLL will lock.

Devicetree

The devicetree register max_mem_clock_mhz overrides the “soft” fuses set by the board manufacturer.

By using this register it’s possible to force a minimum operating frequency.

Reference clock

While Sandy Bridge supports 133 MHz reference clock (REFCK), Ivy Bridge also supports 100 MHz reference clock. The reference clock is multiplied by the DRAM multiplier to select the DRAM frequency (SCK) by the following formula:

REFCK * MULT = 1 / DCK

Note: Since coreboot 4.6 Ivy Bridge supports 100MHz REFCK.

Sandy Bridge’s supported frequencies

SCK [Mhz]	DDR [Mhz]	Mutiplier (MULT)	Reference clock (REFCK)
400	DDR3-800	3	133 MHz
533	DDR3-1066	4	133 MHz
666	DDR3-1333	5	133 MHz
800	DDR3-1600	6	133 MHz
933	DDR3-1866	7	133 MHz
1066	DDR3-2166	8	133 MHz

Ivy Bridge’s supported frequencies

SCK [Mhz]	DDR [Mhz]	Mutiplier (MULT)	Reference clock (REFCK)	Comment
400	DDR3-800	3	133 MHz
533	DDR3-1066	4	133 MHz
666	DDR3-1333	5	133 MHz
800	DDR3-1600	6	133 MHz
933	DDR3-1866	7	133 MHz
1066	DDR3-2166	8	133 MHz
700	DDR3-1400	7	100 MHz	‘1
800	DDR3-1600	8	100 MHz	‘1
900	DDR3-1800	9	100 MHz	‘1
1000	DDR3-2000	10	100 MHz	‘1
1100	DDR3-2200	11	100 MHz	‘1
1200	DDR3-2400	12	100 MHz	‘1

‘1: since coreboot 4.6

Multiplier selection

coreboot selects the maximum frequency to operate at by the following formula:

if devicetree's max_mem_clock_mhz > 0:
     freq_max := max_mem_clock_mhz
else:
     freq_max := soft_fuse_max_mhz

for i in SPDs:
     freq_max := MIN(freq_max, ddr_spd_max_mhz[i])

As you can see, by using DIMMs with different maximum DRAM frequencies, the slowest DIMMs’ frequency will be selected, to prevent over-clocking it.

The selected frequency gives the PLL multiplier to operate at. In case the PLL locks (see Take me to Hard fuses) the frequency will be used for all DIMMs. At this point it’s not possible to change the multiplier again, until the system has been powered off. In case the PLL doesn’t lock, the next smaller multiplier will be used until a working multiplier will be found.